In elevator systems and escalator systems, movement of the transport appliance is permitted only when the preconditions required to ensure the safety of passengers are fulfilled. For example, in elevator systems movement of the elevator car is permitted only when the doors of the elevator car and of the shaft are closed. In elevator systems and escalator systems safety is typically ensured with a safety circuit. The safety circuit can be implemented e.g. such that switches, which are connected to each other in series, are placed in the points that are essential from the standpoint of safety. The electricity supply of the motor of the transport appliance and in an elevator system the opening of the holding brakes are only permitted if all the switches of the safety circuit are closed.
Normally at least the coil of the main contactor and the coil of the machinery brake of the motor are in the same circuit with the switches. The circuit of the switches is arranged to open in a dangerous situation, in which case the main contactors open and the machinery brake energizes. The status of the safety circuit can also be monitored with the control system, e.g. by measuring the voltage across the circuit of the switches according to prior art.
In order to locate a dangerous situation, the status of individual switches in the elevator system or escalator system must be measured. For this purpose at least some of the switches of the safety circuit are conventionally wired separately to the control system for measuring the statuses of individual switches. The control system can be disposed in the machine room or e.g. on the landing floor of the elevator, and the switches can be situated at a distance from the control system, such as in the elevator shaft or in the elevator car. In this case wiring individual switches to the control system substantially increases the amount of wiring.
If individual switches are not monitored and their operation is not supervised either, there must otherwise be safeguards for the safe operation of the circuit formed by the switches. In this case the switch must be constructed as a duplicated switch that opens under forced control. This kind of switch of special construction is expensive.
Prior art technology is represented in publication elevator US-20040173410, which contains an arrangement for monitoring the door switches of an elevator system. Each door switch is monitored separately and the status data of the switches is transmitted to a serial interface bus. For adequate reliability to be achieved with this kind of solution, the measurements of the switches as well as the serial interface bus and the electronics participating in the serial interface communication must be duplicated. This increases the costs of the overall system. In addition, if it is desired to connect different actuators to the same system, also the controls of these actuators as well as their monitoring must be duplicated in order to achieve adequate reliability.
Publication JP 9-2764 A presents an arrangement for monitoring the safety circuit of an elevator. The arrangement comprises a control appliance, and the safety circuit comprises as serial circuit of switches. The arrangement comprises means for measuring the status of at least one switch, as well as means for conveying the status information of the switch to the control appliance.